docs: add NOTICE.md detailing project modifications

NOTICE.md

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+# FLVX - Modification Notice
+
+This project is a derivative work based on **flux-panel** (https://github.com/bqlpfy/flux-panel).
+
+## Original Project
+- **Name**: flux-panel
+- **Source**: https://github.com/bqlpfy/flux-panel
+- **License**: Apache License 2.0
+
+## Modifications
+The following major changes and additions have been made in this fork (FLVX):
+
+### 1. Backend Architecture (Replaced)
+- **Removed**: The original `springboot-backend/` (Java/Spring Boot) has been entirely removed.
+- **Added**: A new `go-backend/` (Go/SQLite) implementation replaces the original backend.
+
+### 2. Forwarding Agent (Modified)
+- **Modified**: `go-gost/` - Modified forwarding agent wrapper.
+- **Modified**: `go-gost/x/` - Modified local fork of the `gost` extensions library.
+
+### 3. Frontend (Modified)
+- **Modified**: `vite-frontend/` - Significant updates to the React/Vite dashboard to compatible with the new Go backend, including UI/UX improvements (HeroUI + Tailwind).
+
+### 4. Mobile Applications (Removed)
+- **Removed**: `android-app/` - Source code for the Android client.
+- **Removed**: `ios-app/` - Source code for the iOS client.
+
+### 5. Infrastructure & Scripts
+- **Modified**: `docker-compose-v4.yml`, `docker-compose-v6.yml` (Updated for Go backend).
+- **Modified**: `install.sh`, `panel_install.sh` (Updated installation logic).
+- **Added**: `AGENTS.md` (Project documentation).
+
+## License for Modifications
+All new files, directories, and modifications introduced in this fork are licensed under the **GNU General Public License v3.0 (GPLv3)**.
+
+> This program is free software: you can redistribute it and/or modify
+> it under the terms of the GNU General Public License as published by
+> the Free Software Foundation, either version 3 of the License, or
+> (at your option) any later version.
+
+For the full text of the GPLv3, see <https://www.gnu.org/licenses/gpl-3.0.html>.